The need for high-speed broadband packet services will grow tremendously as telecommuting and Internet access become increasingly popular. Customers will expect high quality, reliable access to high-speed communications from homes and small businesses in order to, for example, access: (a) the World Wide Web for information and entertainment; (b) office equipment and data from home at rates comparable to Local Area Networks (LANs); and (c) multimedia services such as voice, image and video. Although varying with application, effective broadband communication requires a bandwidth sufficient to permit a data rate up to the range of several tens of Mega-bits per second (Mbps).
Traditional wireless communications systems have a problem delivering high-speed services because of the amount of bandwidth these services require. Bandwidth is a key limiting factor in determining the amount of information a system can transmit to a user at any one time. In terms of wireless networks, bandwidth refers to the difference between the two limiting frequencies of a band expressed in Hertz (Hz).
The concept of bandwidth may be better understood using an analogy. If information carried by a network were water, and links between communication sites were pipes, the amount of water (i.e., information) a network could transmit from one site to another site would be limited by the speed of the water and the diameter of the pipes carrying the water. The larger the diameter of the pipe, the more water (i.e., information) can be transmitted from one site to another in a given time interval. Likewise, the more bandwidth a communications system has available to it, the more information it can carry.
Traditional wired communications systems using modems and a physical transmission medium, such as twisted pair copper wire, cannot currently achieve the data rates necessary to deliver high-speed service due to bandwidth limitations (i.e., small pipes). Promising wired-network technologies for broadband access, such as Asymmetrical Digital Subscriber Loop (ADSL) and Hybrid Fiber-Coax (HFC), can be expensive and time consuming to install.
The benefit of wireless systems for delivering high-speed services is that they can be deployed rapidly without installation of local wired distribution networks. However, traditional wireless systems such as narrowband cellular and Personal Communications Services (PCS) are bandwidth limited. As an alternative, wireless solutions such as Multichannel Multipoint Distribution Service (MMDS) and Local Multichannel Distribution Service (LMDS) have become attractive, but these solutions presently offer limited uplink channel capacity and may not be capable of supporting a large number of users.
One solution for solving the bandwidth limitation problem for wireless systems is to maximize the available bandwidth through frequency reuse. Frequency reuse refers to reusing a common frequency band in different cells within the system. Refer, for example, to FIG. 1 which shows a typical wireless communication system. A Base Station (BS) 20 communicate with several Terminal Stations (TS) 22. The BS 20 is usually connected to a fixed network 24, such as the Public Switched Telephone Network (PSTN) or the Internet. The BS 20 could also be connected to other base stations, or a Mobile Telephone Switching Office (MTSO) in the case of a mobile system. Each TS 22 can be either fixed or mobile.
The BS 20 communicates information to each TS 22 using radio signals transmitted over a range of carrier frequencies. Frequencies represent a finite natural resource, and are in extremely high demand. Moreover, frequencies are heavily regulated by both Federal and State governments. Consequently, each cellular system has access to a very limited number of frequencies. Accordingly, wireless systems attempt to reuse frequencies in as many geographic areas as possible.
To accomplish this, a cellular system uses a frequency reuse pattern. A major factor designing a frequency reuse pattern is the attempt to maximize system capacity while maintaining an acceptable Signal-to-Interference Ratio (SIR). SIR refers to the ratio of the level of the received desired signal to the level of the received undesired signal. Co-channel interference is interference due to the common use of the same frequency band by two different cells.
To determine frequency reuse, a cellular system takes the total frequency spectrum allotted to the system and divides it into a set of smaller frequency bands. A cellular communications system has a number of communications sites located throughout a geographic coverage area serviced by the system. The geographic area is organized into cells and/or sectors, with each cell typically containing a plurality of communications, sites such is a base station and terminal stations. A cell can be any number of shapes, such as a hexagon. Groups of cells can be formed, with each cell in the group using a different frequency band. The groups are repeated to cover the entire service area. Thus, in essence, the frequency reuse pattern represents the geographic distance between cells using the same frequency bands. The goal of a frequency reuse pattern is to keep co-channel interference below a given threshold and ensure successful signal reception.
The most aggressive frequency reuse pattern is where the same frequency band is use in every cell. One example of such a system is Code Division Multiple Access (CDMA) systems, which spread the transmitted signal across a wide frequency band using a code. The same code is used to recover the transmitted signal by the CDMA receiver. Although CDMA systems reuse the same frequencies from cell to cell, they require a large amount of frequency spectrum. In fact, the amount of spectrum required by CDMA systems to offer high-speed broadband services to a large number of users is commercially unrealistic.
Another example for aggressive frequency reuse Time Division Multiple Access (TDMA) systems, an example of which is discussed in U.S. Pat. No. 5,355,367, which use the redundant transmission of information packets to ensure an adequate SIR. The of redundant packet transmissions, however, merely trades one inefficiency for another. Although a frequency band can be reused from cell to cell, redundant packet transmission means that a smaller portion of that frequency band is now available for use by each cell in the system since multiple packets are required to ensure the successful reception of a single packet.
In addition to the frequency reuse problem, traditional cellular systems, are not engineered to allow a communications site to use the entire bandwidth available to the system (or "total system bandwidth"). Rather, traditional cellular systems employ various techniques in both frequency domain and time domain to maximize the number of users capable of being serviced by the system. These techniques are predicated on allocating smaller portions of the total system bandwidth to service individual communication sites. These smaller portions are incapable of providing sufficient bandwidth to offer high speed services.
An example of a technique employed in the frequency domain is Frequency Division Multiple Access (FDMA). FDMA splits the available bandwidth into smaller sections of bandwidth under the concept of providing less bandwidth for a greater number of users. Using the water/pipe analogy, a single large pipe is separated into a number of smaller pipes, each of which is assigned to a sector or cell. Unfortunately, the smaller frequency bands are too small to support high-speed broadband packet services. Moreover, by definition, a communication site is not capable of using the total system bandwidth, but rather is limited to a discrete portion of the total system bandwidth.
An example of a technique employed in the time domain is TDMA, described above. Using the water/pipe analogy, each cell or sector has access to the entire pipe for a fixed amount of time. These systems allocate a specific time slot of a fixed duration for a specific communication site. As a result, a communication site cannot transmit more information than can be accommodated by its assigned time slot. Traditional TDMA systems are designed to handle circuit switching and, therefore, are static in nature. Thus, traditional TDMA systems are not designed to take advantage of new switching technology, such as packet switching.
Some systems employ a combination of FDMA and TDMA to improve the call capacity of the system. These FDMA/TDMA systems, however, merely combine the disadvantages of both and do not permit a user access to the total system bandwidth on a dynamic basis. To solve this problem, some systems employ a concept called "dynamic resource allocation" to share the radio resource among communications sites efficiently. Dynamic resource allocation methods, however, require a central controller or complicated algorithms to dynamically determine available time slots and coordinate their use by the communication sites.
In order to increase spectrum efficiency, other cellular systems have employed multiple frequency reuse patterns within the same system. For example, U.S. Pat. No. 4,144,411 issued to Frenkiel on Mar. 13, 1979, teaches static reuse of frequencies in a system employing a miniature-sized overlay in each cell, with the miniature-sized overlay using the same type of reuse pattern as the large cell reuse pattern. This is achieved through yet lower transmit powers and maintaining the same site spacing to cell radius as the large-cell. This concept is typically referred to as cell splitting.
An enhancement to Frenkiel is discussed in an article authored by Samuel W. Halpern entitled Reuse Partitioning in Cellular Systems, presented at the 33rd IEEE Vehicular Technology Conference on May 25-27, 1983 in Toronto, Ontario, Canada. The, Halpern article sets forth a cellular system having multiple frequency reuse levels (or patterns) within a given geographical area. For example, a cluster of cells normally employing a seven-cell reuse pattern may simultaneously operate on a three-cell reuse pattern and a nine-cell reuse pattern. One set of frequencies is dedicated to the three-cell reuse pattern while another set of frequencies is dedicated to the nine-cell reuse pattern. Generally, the principle behind the Halpern system is to allow a degradation of Carrier-to-Interference (C/I) performance for those subscriber units that already have more than adequate C/I protection while providing greater C/I protection to those subscribers that require it. Therefore, a subscriber with the best received signal quality will be assigned to the set of channels for the three-cell reuse pattern since they are able to tolerate more co-channel interference than a subscriber whose signal quality is poorer. The subscriber having the poorer received signal quality is therefore assigned to a channel correspondent to the nine-cell reuse pattern.
The Halpern system, like previous multiple frequency reuse partitioning systems, is unsatisfactory for a number of reasons. For example, in practice the Halpern system permits only a small fraction of the total traffic to use the closer reuse pattern for the miniature-sized overlay, leading to little or no gain in system capacity. Further, the Halpern s system is designed for circuit switched systems, and not for the modern packet switched systems. More specifically, circuit switched systems can tolerate a lot of measurement overhead and delaying when connecting to the user. If the same techniques were applied to a packet switched system, however, several measurements would be required before transmitting each packet. The overhead and delay introduced would be excessive, and therefore the method described in the Halpern reference would not be feasible. In fact, the Halpern method is designed for the conversational telephony system, and not packet switched systems in general.
Moreover, previous systems were designed to do the reuse partitioning in the frequency domain, that is they were focused on dividing the total frequency bandwidth available to the system and allocating one portion of this total frequency bandwidth to one reuse pattern, and another portion to another reuse pattern. Dividing the available frequency, however, limits the maximum data rate that can be provided to any single user or application by the system. Therefore, frequency reuse partitioning schemes are not suitable for supporting high data rate applications such as those envisioned for wireless broadband systems.
A specific implementation of frequency reuse partitioning is disclosed in U.S. Pat. No. 5,038,399 (the "Bruckert patent"). The Bruckert system is directed towards a mechanism for measuring various signal strengths from base stations and subscriber stations throughout the system, constructing a reuse level gradient, and using this gradient as a basis for switching between multiple frequency reuse patterns.
As with the Halpern system, the Bruckert system is unsatisfactory for a. number of reasons. For example, the Bruckert patent is also targeted towards a circuit switched system and is not designed towards modem packet switched systems. As a result, the bandwidth available to a user is fixed during the call duration, thus becoming inflexible for handling data bursts as anticipated in broadband services. Furthermore, the Bruckert patent describes a method for assigning different users to different reuse levels according to the "reuse level gradient," which is another way of stating the assignment is based upon different interference levels. In many instances, however, an integrated system providing different services to the same user may require different reuse levels due to their differing service requirements, even though they experience the same interference. The Bruckert patent fails to disclose how the quality of service (QoS) is maintained for each application using this method. In addition, the Bruckert patent fails to disclose any techniques for ensuring fairness among communication sites in terms of each site gaining access to the communication resource in a uniform manner. Finally, the Bruckert patent fails to disclose the use of multiple reuse patterns in the time domain, as with the previously discussed systems.
In view of the foregoing, it can be appreciated that a substantial need exists for a dynamic resource allocation method and apparatus for broadband services in a wireless communications system that efficiently provides high quality broadband packet services to a large number of users, and solving the other problems discussed above.